compositor-wayland: clarify wl_display_connect() error

compositor-wayland: clarify wl_display_connect() error

Clarify the error message to explicitly say one was trying to connect to
a parent Wayland compositor. This hopefully is a good enough hint on
what using the wayland-backend is trying to do.

Add the command line display option value and WAYLAND_DISPLAY values for
good measure. WAYLAND_SOCKET is not shown as libwayland-client removes
it.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Acked-by: Daniel Stone <daniels@collabora.com>